ac problem

Pressure build on the low side?

yes

Without knowing the high pressure side, it is not possible to pin point the exact fault.

Possible causes are :
a) If low pressure hose and check joint are cooler than around evaporator, the expansion valve is faulty.

b) If the suction pressure is lowered when condenser is cooled by water, you have excessive referigerant in system.

c) If high and low pressure equalized as soon as compressor is stopped, the compressor is faulty.

Like i said before you need both pressures for diagnoses.

the high side is 40 and the low side is 80 withon can of freon

That is not possible. Reonfirm the pressure reading.

i had a small garage look at it and put a full charge in it. it 200# in high side and 80 in low side.he thinks the compressor is not sucking in.thought it should be 40# in low side

The low side is too high whereas the high side is below expectation so a faulty compressor is the likely cause.

thank you